Move Field

Before you begin: Objects, fields, constructors, and properties.

Move Field relocates stored data to a more appropriate object and redirects the code that reads or writes it. The question is which object should own the value, not which file has spare room. 1

Original version: an encoding setting is stored on the video

A video has a title and encoding settings. Its codec is in an encoding object, but its bitrate is stored separately on the video.

public sealed class EncodingOptions
{
    public string Codec { get; } = "opus";
}

public sealed class Video
{
    public string Title { get; }
    private readonly int bitrate;
    private readonly EncodingOptions encoding = new EncodingOptions();
    public Video(string title, int bitrate)
    {
        Title = title;
        this.bitrate = bitrate;
    }
    public string EncodingLabel()
    {
        return encoding.Codec + " " + bitrate;
    }
}

class EncodingOptions:
    codec = "opus"


class Video:
    def __init__(self, title, bitrate):
        self.title = title
        self._bitrate = bitrate
        self._encoding = EncodingOptions()

    def encoding_label(self):
        return self._encoding.codec + " " + str(self._bitrate)

The label combines two encoding facts stored in different objects. Any operation that needs the complete encoding choice must collect those facts again.

Refactored version: keep the encoding facts together

public sealed class EncodingOptions
{
    public string Codec { get; } = "opus";
    public int Bitrate { get; }
    public EncodingOptions(int bitrate) { Bitrate = bitrate; }
}

public sealed class Video
{
    public string Title { get; }
    private readonly EncodingOptions encoding;
    public Video(string title, int bitrate)
    {
        Title = title;
        encoding = new EncodingOptions(bitrate);
    }
    public string EncodingLabel()
    {
        return encoding.Codec + " " + encoding.Bitrate;
    }
}

The constructor still accepts the same arguments, but stores bitrate in EncodingOptions. EncodingLabel reads it from that new location.

class EncodingOptions:
    codec = "opus"

    def __init__(self, bitrate):
        self.bitrate = bitrate


class Video:
    def __init__(self, title, bitrate):
        self.title = title
        self._encoding = EncodingOptions(bitrate)

    def encoding_label(self):
        return self._encoding.codec + " " + str(self._encoding.bitrate)

The public construction and label result stay the same. Bitrate now lives with the codec it configures.

Keep one authoritative value

Do not leave independently writable copies in both objects. During migration, an old property can forward to the new field; two separate values can drift apart.

Check whether the receiving object is shared. Moving a per-video setting into one encoding object used by many videos could unintentionally make them share future edits. This example creates an encoding object for each video.

Stored schemas, serialization, and external consumers may also need migration. Moving an in-memory field is not automatically a safe database change. The useful result is clearer ownership with the same required behavior, not simply a smaller class.

Sources

Martin Fowler — Move Field.